Polyether alcohols are used in large amounts for producing polyurethanes. Their preparation is usually carried out by catalytic addition of lower alkylene oxides, in particular ethylene oxide and propylene oxide, onto H-functional initiator substances. Catalysts used are usually basic metal hydroxides or salts, with potassium hydroxide having the greatest practical importance.
In the synthesis of polyether alcohols having long chains, as are used, in particular, for producing flexible polyurethane foams, as chain growth progresses it is associated with secondary reactions which lead to faults in the chain structure. These by-products are known as unsaturated constituents and lead to an impairment of the properties of the resulting polyurethanes. There have therefore been many attempts in the past to prepare polyether alcohols having a low content of unsaturated constituents. For this purpose, in particular, the alkoxylation catalysts used are altered in a targeted way. Thus, EP-A-268 922 proposes using cesium hydroxide. Although this can lower the content of unsaturated components, cesium hydroxide is expensive and presents a disposal problem.
Furthermore, the use of multimetal cyanide complexes, mostly zinc hexacyanometalates, for the preparation of polyether alcohols having low contents of unsaturated constituents is known. There is a large number of documents in which the preparation of such compounds is described. Thus, DD-A-203 735 and DD-A-203 734 describe the preparation of polyetherols using zinc hexacyanocobaltate.
The preparation of the zinc hexacyanometalates is also known. These catalysts are usually prepared by reacting solutions of metal salts such as zinc chloride with solutions of alkali metal or alkaline earth metal cyanometalates such as potassium hexacyanocobaltate. In general, immediately after the precipitation procedure, a water-miscible, heteroatom-containing component is added to the precipitation suspension obtained. This component can also be present beforehand in one or both starting solutions. This water-miscible, heteroatom-containing component can be, for example, an ether, polyether, alcohol, ketone or a mixture thereof. Such processes are described, for example, in U.S. Pat. No. 3,278,457, U.S. Pat. No. 3,278,458, U.S. Pat. No. 3,278,459, U.S. Pat. No. 3,427,256, U.S. Pat. No. 3,427,334, U.S. Pat. No. 3,404,109, U.S. Pat. No. 3,829,505, U.S. Pat. No. 3,941,849, EP 283,148, EP 385,619, EP 654,302, EP 659,798, EP 665,254, EP 743,093, U.S. Pat. No. 4,843,054, U.S. Pat. No. 4,877,906, U.S. Pat. No. 5,158,922, U.S. Pat. No. 5,426,081, U.S. Pat. No. 5,470,813, U.S. Pat. No. 5,482,908, U.S. Pat. No. 5,498,583, U.S. Pat. No. 5,523,386, U.S. Pat. No. 5,525,565, U.S. Pat. No. 5,545,601, JP 7,308,583, JP 6,248,068, JP 4,351,632 and U.S. Pat. No. 5,545,601.
DD-A-148 957 describes the preparation of zinc hexacyanoiridate and its use as a catalyst in the preparation of polyether alcohols. One of the starting materials used here is hexacyanoiridic acid. This acid is isolated as a solid and is used in this form.
A disadvantage of using zinc hexacyanoiridate is its color. The polyether alcohols prepared using this catalyst are usually also slightly yellowish, which for many applications is regarded as a quality defect.
Furthermore, this process cannot be applied to the preparation of other two-metal cyanide complexes, in particular the substantially less expensive cyanocobaltates, since cyanocobaltic acid is substantially less stable and is virtually impossible to handle as a solid.
A disadvantage of processes starting from cyanometalate salts is that they form not only the desired multimetal cyanide complex catalyst but also an unavoidable amount of salt, eg. potassium chloride when zinc chloride and potassium hexacyanocobaltate are used, which has to be removed from the catalyst in order to achieve a high activity. Since the addition of the organic components to the precipitation suspension considerably reduces the solubility of the salts to be removed, the generally very finely divided catalyst has to be washed a number of times with the organic component. In the production of multimetal cyanide complex catalysts, this takes a considerable amount of time and leads to losses of solid, which can be prohibitive for the industrial preparation of such catalysts.
It is an object of the present invention to find a process for preparing multimetal cyanide complex catalysts which does not produce an additional amount of solid and is thus simpler to carry out and leads to catalysts having a high activity.